Heart failure (HF) is a condition where the heart does not contract and relax properly, resulting in inefficient pumping action, usually due to dysfunction of a ventricle (the lower chambers of the heart). HF impairs the ability of the heart to supply sufficient blood flow to meet the needs of the body. As HF progresses, pumping efficiency decreases and the body attempts to compensate by increasing the heart muscle mass. Such compensation is commonly referred to as remodeling. The heart eventually requires more oxygen than it can obtain, which results in further damage to the heart muscle. As remodeling progresses, the walls of the heart stiffen and pumping efficiency decreases. Production of various neuro-hormones is another compensation mechanism that can cause further heart damage.
HF can cause a large variety of symptoms such as dyspnea (shortness of breath), exercise intolerance, fluid retention, ankle swelling, and coughing. The rapid accumulation of fluid within pulmonary interstitial and alveolar spaces results from elevated cardiac filling pressures (cardiogenic pulmonary edema). HF patients can experience heart rhythm abnormalities and lack of ventricle coordinated contractions.
HF is also associated with significantly reduced physical and mental health, resulting in a markedly decreased quality of life and is also characterized by high mortality, frequent hospitalization, and a complex therapeutic regimen. Management of HF is challenging given the heterogeneity of the patient population, absence of a universally accepted definition, incomplete understanding of the pathophysiology, and lack of robust evidence-based guidelines. The majority of patients appear to respond well to initial therapies including loop diuretics and vasoactive agents. However, post-discharge and rehospitalization rates and HF-related mortality within 3-6 months reach 20-30% and 10-20%, respectively (reflecting both the severity of HF and underlying myocardial injury, renal impairment and other diseases).
Heart failure is often undiagnosed due to the lack of a universally agreed definition and challenges in definitive diagnosis. Treatment commonly involves lifestyle measures (such as decreased salt intake) and medications, and sometimes devices or even surgery. With the exception of heart failure caused by reversible conditions, the condition usually worsens with time and is associated with an overall annual mortality rate of 10%.
There are an estimated 5 million patients currently suffering from HF in the US, with a burden of about 1 million US hospitalizations and direct and indirect costs over $35 billion annually. HF has increased significantly over the past decades and is likely to increase further with better treatments for myocardial infarction, an aging population, and improved strategies for sudden cardiac arrest prevention.
Given the huge economic impact of HF hospitalizations, many strategies have been used to prevent inpatient stays including direct patient visits by physicians or other healthcare providers, patient disease management systems (including daily weights, scales, symptom assessment, etc.), and implantable devices using hemodynamic measurements or trans-thoracic impedance as surrogate markers of decompensating HF. The methods and apparatus described herein can be used in a non-invasive fashion, or along with other invasive methods, to provide increased monitoring and diagnosis utility.
To determine the best course of therapy, physicians often assess the stage of heart failure according to the New York Heart Association (NYHA) functional classification system. This system relates symptoms to everyday activities and the patient's quality of life. The table below shows this well-known classification.
TABLE 1Classes of heart failure stages and their associated patient symptoms.ClassPatient SymptomsClass I No limitation of physical activity. Ordinary physical (Mild)activity does not cause undue fatigue, palpitation, or dyspnea (shortness of breath).Class II Slight limitation of physical activity. Comfortable at (Mild)rest, but ordinary physical activity results in fatigue, palpitation, or dyspnea.Class IIIMarked limitation of physical activity. Comfortable at rest, (Moderate)but less than ordinary activity causes fatigue, palpitation,or dyspnea.Class IV Unable to carry out any physical activity without discomfort.(Severe)Symptoms of cardiac insufficiency at rest. If any physical activity is undertaken, discomfort is increased.